The present disclosure relates to a battery separator interposed between a positive electrode and a negative electrode, a method of manufacturing a battery separator, a battery including the battery separator, a battery pack, and an electronic apparatus.
For example, a generally-used lithium-ion secondary battery includes a positive electrode containing a lithium composite oxide, a negative electrode containing a material capable of absorbing and releasing lithium ions, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution. The positive electrode and the negative electrode are laminated on each other via the separator or wound after the lamination, to thereby form a columnar wound electrode. The separator plays a role in electrically isolating the positive electrode and the negative electrode from each other and a role in holding the non-aqueous electrolyte solution. As the separator of such a lithium-ion secondary battery, a microporous polyolefin membrane is generally used.
The microporous polyolefin membrane exhibits excellent electrical insulation property and ion permeability and therefore it has been widely used as a separator of the lithium-ion secondary battery described above, a capacitor, and the like. The lithium-ion secondary battery has high power density and high capacitance density. However, since the lithium-ion secondary battery uses an organic solvent for a non-aqueous electrolyte solution, the non-aqueous electrolyte solution may be decomposed due to heat generated under abnormal conditions such as short circuit and overcharge, which may lead to ignition in the worst case. To prevent such a situation, some safety functions are incorporated into the lithium-ion secondary battery, one of which is a shutdown function of a separator.
The shutdown function of a separator is a function of, when a battery generates abnormal heat, clogging minute pores of the separator by thermal fusion or the like of a resin material and suppressing ion conduction in the non-aqueous electrolyte solution, to thereby stop the progress of an electrochemical reaction. Generally, a lower shutdown temperature provides higher safety, and one of reasons why polyethylene is used as a component of a separator is that polyethylene has an appropriate shutdown temperature. In such a separator, for example, a resin film uniaxially or biaxially stretched is used so as to obtain porous property and improve strength.
In the case of shutdown, however, the separator is shrunk, and therefore the positive electrode and the negative electrode come into contact with each other, which may cause a secondary failure such as internal short circuit. Therefore, it has been demanded to reduce thermal contraction by improving heat resistance of the separator, and thus to improve safety.
For example, Japanese Patent Application Laid-open No. 2009-16279 discloses a separator including a covering layer in which a fine skeleton of a polyolefin resin is covered with a glass layer. Further, Japanese Patent No. 3797729 discloses a battery separator in which an inorganic thin film is formed by a sol-gel method on the surface of a polyolefin porous film without clogging a hole.